When it comes to conditional sentences dealing with the past, we're used to seeing the so-called Third Conditional pattern very often.
If you had heard his remark, you would have felt embarrassed.
It's about a remark that you didn't hear and, therefore, it's clear that you didn't feel embarrassed either. The condition (which is in the if-clause) didn't happen, so the possible result of it could not happen. One is trying to visualise a scenario that was possible, but didn't really occur. That's where we use this pattern called the third conditional in English.
Sometimes we have to talk about a past condition we aren't quite sure about; that is, we don't know for sure whether it really happened as claimed by someone. This is a somewhat different scenario, and a different pattern has to be employed to convey this type of idea.
If you heard his remark, you must have felt embarrassed.
If you heard his remark, why didn't you respond?
In the above two cases, the other person may have claimed that they heard the remark, and the sentences are about possible results that depend on the veracity of that claim.
As conventional grammar books tend to focus only on three main types of conditional sentences, some students of English find this second pattern (which isn't one of those three) a bit confusing. This is further complicated by the nature of the second conditional pattern, which mostly uses past-tense verbs in the if-clause to refer to an improbable future (or present) occurrence.
If you heard a remark like that, you would feel embarrassed.
The above example has nothing to do with the past. It's all about the present and the future. Maybe the speaker thinks the chances are rather slim that the other person will hear such a remark; or it could be an attempt at expressing one's opinion in a more polite way. Anyway the speaker is of the opinion that hearing this kind of remark is likely to cause embarrassment to the other person.
The term 'if', referring to the past, could also have the meaning 'whenever' (and the pattern would be similar to the second conditional):
If she saw me, she would smile.
(Whenever she saw me, she smiled.)
Or it may even mean something like 'though...(admittedly)' :
If that boy was silent, he certainly wasn't dumb.
(Though that boy was (admittedly) silent, he certainly wasn't dumb.)
Related post: If She Found Out About it...
(Image credit: antonpinchuk)